1. Field of the Invention
The present invention relates to a structure for a sealed neutron tube generating fast neutrons used in measurements, such as oil well logging and the like, and in particular to a high voltage insulating structure, an ion source structure and a target structure of the sealed neutron tube.
2. Description of the Related Art
A high voltage insulating structure, an ion source structure and a target structure of a sealed neutron tube generating fast neutrons which is used in measurements, such as oil well logging and the like, will be described with reference to FIGS. 4 to 8.
As shown in FIG. 4, a sealed neutron tube 1 includes a cylindrical housing wall 2, a target 3 absorbed Deuterium or Tritium therein inserted into the housing wall 2, an accelerating electrode 4 charged with a high voltage, and an ion source 5 for ionizing the Deuterium gas. A reservoir 6 in which Deuterium is filled is attached to the ion source 5. The target 3 is made contact through a high voltage connecting rod 7 to a high voltage power supply (not shown).
The operating principle with the sealed neutron tube 1 thus constructed will now be described. The Deuterium ions discharged in the ion source 5 are accelerated by the electric potential formed between the ion source 5 and the accelerating electrode 4 to collide with the target 3. This collision causes nuclear fusion reaction between the Tritium or the Deuterium absorbed in the target 3 and the Deuterium ions accelerated with about 100 kV, to thereby generate neutrons.
Here, in order to accelerate the Deuterium ions extracted from the ion source 5 to the accelerating electrode 4 within the sealed neutron tube 1, an accelerating voltage of around 100 kV is applied between the ion source 5 and the accelerating electrode 4 (namely, the target 3). For this reason, in a case where the ion source 5 side has a grounded electric potential, the accelerating electrode 4 must be supported on the housing wall 2 in an electrically insulated manner.
Therefore, as shown in FIG. 4, the housing wall 2 of the sealed neutron tube 1 is constructed by an insulating member 8 formed of a ceramic or glass, and the accelerating electrode 4 (namely the target 3) and the ion source 5 are supported by this housing wall 2 to provide an insulating structure. In FIG. 5, an alternative insulating structure for the sealed neutron tube 1 is shown, in which only a part of the housing wall 2 (i.e., an accelerating electrode 4 side of the housing wall 2) is constructed by the insulating member 8 formed of ceramic or glass to insulate the accelerating electrode 4 from the housing wall 2.
Moreover, secondary electrons are emitted from the target 3 upon the collision of the Deuterium ion beam, extracted and accelerated from the ion source 5, with the target 3. These secondary electrons are attracted to a grounded electric potential portion, such as the ion source 5, due to the electric potential existing between the ion source 5 and the accelerating electrode 4. Since a current which flows due to this secondary electron emission results in a loss of energy which does not contribute to the generation of neutrons, a structure is required to suppress the secondary electrons emitted from the accelerating electrode 4.
Hence, as shown in FIG. 4, in order to suppress the secondary electrons emitted from the accelerating electrode 4, a Faraday cap structure for the accelerating electrode 4 is accommodated in the sealed neutron tube 1. That is, the target 3 is enveloped by the accelerating electrode 4, and the electric potential of the accelerating electrodes 4 is less than that of the target 3 by about 500 to 2000 V. With the Faraday cap structure constructed in this manner, the ion beam is collided to the target 3 while being accelerated by the electric potential formed between the ion source 5 and the accelerating electrode 4, whereas the secondary electrons are returned to the target 3 side by the electric potential formed between the accelerating electrode 4 and the target 3 and are suppressed to be released toward the ion source 5 side.
Next, an example of a cold cathode type ion source will be described with reference to FIGS. 6 and 7. FIG. 6 illustrates an example of the ion source 5 using a cylindrical magnet 51. The ion source 5 includes cathodes 52 attached to the ends of the cylindrical magnet 51, and a cylindrical anode 53 disposed among the cathodes 52. A plasma generating section 55 is defined in a space enveloped by the magnet 51, the cathodes 52 and the anode 53. The cylindrical anode 53 is connected to a pulsed power supply 54. An ion outlet hole 56 is disposed at the cathode 52 where the ion source 5 faces the target 3.
The generating principle of plasma with the ion source 5 thus constructed will now be described. Initially, a magnetic potential is formed in the axial direction by the magnet 51 within the ion source 5 and a voltage of 1 to 3 kV is applied to the anode 53. Next, the temperature of the reservoir 6 (see FIG. 4) in which the Deuterium is absorbed is subsequently raised to increase the gas pressure within the sealed neutron tube 1 to about 10.sup.-3 to 10.sup.-2 mmHg. As a result, plasma is generated in the plasma generating space 55 within the ion generating source 5 by the synchronized action of the electric potential formed by the anode 53 and the cathodes 52 and the magnetic potential formed by the magnet 51. The positive ions in the plasma generated in the plasma generating space 55 is extracted out of the ion outlet hole 56 by the electric potential formed between the ion source 5 and the accelerating electrode 4 (see FIG. 4). The positive ions thus extracted of the ion outlet hole 56 form the ion beam and collide with the target 3 (see FIG. 4). In addition, in order to generate the neutrons in a pulse-shaped manner, the plasma is intermittently generated within the ion source 5. To this end, the pulse power supply 54 applies the pulsed voltage to the anode 53 of the ion source 5.
FIG. 7 illustrates an example of the ion source 5 using a rod type magnet 57 as the magnet forming the magnetic potential. In this example as well, the pulsed voltage is applied from the pulsed power supply 54 to the anode 53 of the ion source 5 to intermittently generate the plasma within the ion source 5, thereby generating neutrons in a pulse-shaped manner.
Next, the target 3 which is generally used, will be described with reference to FIG. 8. The target 3 includes a coin-like metal base 31 and a film of metal absorbing hydrogen 32 coated on the metal base 31 by processing such as sputtering of the metal absorbing hydrogen. The film of metal absorbing hydrogen 32 is coated entirely on one side of the metal base 31, or circularly coated on the one side of the metal base 31. The thickness of the film of metal absorbing hydrogen 32 is about a uniform 1 to 10 .mu.m.
The sealed neutron tube to be used for oil well logging requires high shock-proof performance because it operates in a bore hole. The sealed neutron tube 1, however, has a problem in that the insulating housing wall 2 used, which is formed by an insulating member 8 of glass or the like, has insufficient shock-proof performance. In the case of the housing wall 2 formed by an insulating member 8 of ceramic, the ceramic may be dielectrically broken down and perforated by shocks, such that the sealed neutron tube 1 breaks down. Further, the damage to the housing wall 2 will result in leakage of the internally sealed tritium (a radioactive isotope) out of the sealed neutron tube 1. That is, not only does the sealed neutron tube 1 break down, but a serious problem is also caused in safety handling. Therefore, there has been a demand to form the insulating structure of the sealed neutron tube 1 as a firmer structure having sufficient shock-proof properties, and in particular, as such a structure which prevents the Tritium from being externally leaked even if the sealed neutron tube 1 is damaged.
The pulsed neutron generating method adopted for the ion source 5 depends on turning on and off the voltage applied to the anode 53 using the pulse power supply 54. Therefore, it has been known that there is a slight time lag between the application of the voltage to the anode 53 with the pulse power supply 54 turned on and the stabilized generation of the plasma in the plasma generating section 55. This time lag in the sealed neutron tube 1 is about 3 to 10 micro seconds. In contrast, when an inelastic scattering .gamma.-ray is analyzed in oil well logging, as the generated pulse width and pulse shape of the neutron beam generated in the pulse-shaped manner becomes shorter and more accurate, respectively, the accuracy of the logging used pulsed neutron becomes higher. Therefore, the sealed neutron tube 1 used for the oil well logging requires an ion source that is driven at a high speed in order to shorten the interval of the pulse rate of the neutron burst.
Since the target 3 is irradiated by the Deuterium ions and the like extracted from the ion source 5, the film of metal absorbing hydrogen 32 coated on the target 3 may be eroded due to the sputtering effect of the ion beam. Therefore, protection against the shortening lifetime of the target 3 due to this erosion is necessary.
It is conceivable, as a solution to protect against shortening lifetime, to make the film of metal absorbing hydrogen of the target 3 more thick, thereby making the lifetime of the film of metal absorbing hydrogen long. However, since the film of metal absorbing hydrogen in an amount substantially proportional to the increased amount of metal absorbing hydrogen, it is necessary to increase the filling amount of the Deuterium and Tritium in proportion to the thickness of the film of metal absorbing hydrogen 32.
For example, in a case where Ti is used as the metal absorbing hydrogen, the metal absorbing hydrogen Ti can absorb the hydrogen isotopic element (i.e., Deuterium and Tritium in the case of the sealed neutron tube) in a ratio substantially equal to the atomic ratio of the Ti and the hydrogen isotopic element (Ti:hydrogen isotopic element=1:1.8). Accordingly, if the target is 10 .mu.M thick and 12 mm in diameter, the weight of the metal absorbing hydrogen Ti is 2.7 mg, which corresponds to 5.6.times.10.sup.-5 mol, and therefore there is a possibility that the Tritium and Deuterium will be absorbed at 10.1.times.10.sup.-5 mol, that is, 1.8 times the weight. Assuming that the Tritium shares 1/2 of the amount absorbed into the target 3, this corresponds to 1.5 Ci (5.6.times.10.sup.10 Bq).
In contrast, from the standpoint of the waste disposal problem encountered after the sealed neutron tube 1 is used, it is desirable that the amount of Tritium, a radioactive base, used is made as small as possible. Therefore, it is desirable to lengthen the lifetime of the target without substantially increasing the amount of tritium to be absorbed.